#1 has Spell construction rules, in combination with #10 that gives you just about everything you would ever need for Magic.
#2 Adds the Shaman, Sorceror and Warlock and spells for Black Magic, Spirit Magic, and Sorcery also has a section on the planes
#3 has rules on building classes and shows how the base classes were built, also has a bunch of Talents
#4 Adds Beastmaster, Monk, Ranger, and Warrior Mage and their spells
#5 expands on craftsmanship and Herbalism also introduces Training Paths
#6 has Different Spell Drain options, information on Commerce, and alternative ways to resolve actions
#7 has rules for Creating Races and adds several new ones including Centaurs, Gnomes, and Lizardmen
#8 is all about options including Combat Style creation rules, expanded action points, expanded initiative options, etc.
#9 Adds Bard, Mage Hunter, Paladin and Shadowkin classes and their spells.
#10 as previously mentioned has the Magic Item Creation rules
I like #1 because I like the option to create my own spells. I like #4 and #9 because I like to have those sorts of characters available to play. I like #7 because it is easier to make a custom world seem unique if you can add a custom race or two. I don't have #10 yet but I always like building odd magic items.
Hopefully this helps
